Bipotential cathode



Jan. 21, 1964 L. A. HARRIS 3,119,041

BIPOTENTIAL CATHODE Filed Dec. 26. 1961 [r7 ve r2 tor-z- Ld wr-ence A.Har-rvs,

United States Patent Ofi ice 3,119,041 Patented Jan. 21, 1964 3,119,041 BIPGTEN'HAL CATHODE Lawrence A. Harris, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 26, 1961, Ser. No. 161,804 Claims. (Cl. 313-493) The present invention relates to improved bipotential cathodes.

In high vacuum type electric discharge devices employing control grids, electron flow to the positive grid poses a limit on the range of operation of the device. A reduction in this grid current diminishes grid heating and as a result, the probability of grid deformation or destruction. Emission of electrons from the grid is also minimized. Wider ranges of grid voltage are possible with resultant increases in power output capabilities and efiiciency. There have been attempts in the prior art to minimize the collection of electron current by the control grid and in some of these the emitting material on the cathode was simply omitted in areas under the grid wires. In accordance with the invention described and claimed in copending Beggs application Serial No. 118,150, filed June 19, 1961, is described and claimed an improved electric discharge device in which the contact potential difference existing between the areas of the cathode not coated with emitting material and the coated portions of the cathode are utilized to exert a focusing effect on the electrons emitted and in this way to minimize the collection of electrons by the control grid. It is essential to the effective realization of this advantageous result that the contact potential remain throughout the operating life of the device and in accordance with the invention claimed in the aforementioned Beggs application, this is insured by utilizing an active metal for the bare or uncoated portions of the cathode. This metal, such as titanium, has a characteristic of maintaining itself clean, particularly at elevated temperatures, by reacting with the materials that may be deposited thereon and sorbing gases that may be liberated. While the aforementioned invention of Beggs provides an effective solution to this problem, it does impose a limitation that an active material be used at least for the uncoated portions of the cathode. I have found that the advantages of a bipotential cathode may be maintained throughout the life of an electric discharge device of the high vacuum type of utilizing a cathode having emitting areas and areas of bare metal where the bare metal is a very passive material such as platinum. The platinum does not, as a result of its chemical inactivity, exhibit the tendency to maintain itself clean exhibited by the active metal in accordance with the Beggs invention but the emitting material does not have a tendency to migrate over the bare metal surface. This characteristic coupled with the shielding from the deposition of material that is provided by the control grid or otherwise provides an electric discharge device in which the high work func tion of the bare metal is maintained throughout operating life. Accordingly, it is an important object of the present invention to provide a new and improved bipotential cathode.

It is another object of the invention to provide an improved bipotential cathode having bare areas of passive metal of high work function.

In accordance with a preferred embodiment of my invention, I provide a planar type triode high vacuum electric discharge device in which the cathode is made up of a platinum disk having elongated depressions for receiving the electron emission enhancing material and separated by areas or strips of bare platinum. A closely spaced grid electrode having conductors aligned with the bare portions of the cathode and of slightly smaller transverse dimension is provided. With this construction the emitting material does not tend to migrate over the bare platinum area and the shielding provided by the grid electrode also tends to prevent the deposition of such material that may be evaporated from other parts of the interior of the electric discharge device. As a result the contact potential of the bare platinum with respect to the emitting areas is available for focusing the electrons emitted into a relatively well defined cross-section in alignment with the openings in the grid structure.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing:

FIG. 1 is an elevational view, partly broken away, showing a triode electric discharge device embodying my invention, and

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1.

Referring now to the drawing, I have shown my invention embodied in a high vacuum triode type of electric discharge device 10 of generally cylindrical configuration and with planar type electrodes. The envelope of the electric discharge device is made up of a disk-like anode member 11 in a similarly shaped closure disk 12 providing a heater terminal. The cathode is supported from an annular terminal member 13 and the grid is supported from and may be formed integrally with an annular grid terminal member 14. The members 11-14 are sealed together in mutually spaced and insulated relation by annular disks, preferably of ceramic material designated by the numerals 15, 16 and 17. While a number of different materials may be employed for these members, the metal members may to advantage be of titanium and the insulating members 15-17 of a ceramic having temperature expansion characteristics corresponding rather closely to those of titanium. The ceramic may, to advantage, be a forsterite ceramic of the type described and claimed in Pincus Patent 2,912,340, dated November 10, 1959.

In the particular embodiment illustrated, the anode 11 includes a central thickened portion providing an anode surface 18. The grid electrode includes a plurality of generally parallel grid conductors 19 extending in spaced relation from the grid terminal 14 which may be formed integrally with the grid conductors 1 9. The cathode is provided by a disk 2t} bonded to an inwardly directed flange 21 which is axially offset with respect to the oathode terminal 13.

In accordance with an important aspect of the present invention the cathode is provided with surfaces of bare high Work function metal in alignment with the grid conductors and these surfaces are of platinum. While the remainder of the cathode base may be made of any suitable very passive cathode base material, it is a particular advantage that platinum is also very desirable for the cathode base material and this permits a unitary cathode struc ture in which the emitting areas are depressed for the reception of the emitting material so that the bare portions are at least as high as the coated cathode portions. As illustrated in a particular embodiment of my invention shown in the drawing, the cathode disk 2% is of platinum and has formed thereon depressions which may be rolled or stamped into the disk and in which the electron emission enhancing material 2.2 is received therein. These depressions are separated by sunfaces of clean platinum 23 arranged in alignment with grid conductors =19 and are preferably of slightly wider transverse dimension than the grid conductors. The electron emission enhancing material 22 may to advantage be the customary triple carbonate mixture of alkaline earths, i.e., calcium, strontium and barium carbonates, and also in accordance with the preferred embodiment, the coating does not quite fill the depressions so that the edges of the coating lie slightly below the bare interposed bare platinum surfaces 23 and the coated areas are totally devoid of sharp edges which tend to create high fields at the edges thereof. As illustrated in the drawing, the cathode is raised to electron emitting temperature by means of a suitable spiral heater element 24 having one terminal thereof connected with the heater terminal 12 by conductor 25 and the other terminal connected to the cathode terminal 13 by conductor 26.

Instead of utilizing an entire cathode disk of platinum, it is readily apparent that the disk may comprise a composite structure including a thin platinum foil surface and a backing of any suitable material such as nickel or titanium. It is further possible in accordance with my invention in its broader aspects to apply the platinum foil only to the areas where the bare platinum is desired and the actual base for the emitting material may be a very passive cathode metal such as very pure nickel, for example.

In the fabrication of the illustrated embodiment of my invention the cathode may be prepared in accordance with techniques well known in the art and the mixture of carbonates applied to the desired discrete areas which will be in the assembled device in alignment with the openings in the grid structure. The parts are then assembled in a stack in essentially the relationship shown and bonded together in accordance with known methods. For the preferred materials described above, the method of bonding described and claimed in Beggs Patent 2,857,663, dated October 28, 1958, may be employed to advantage. This method employs the use of an interposed shim of suitable material between the titanium and forsterite ceramic, one suitable material being nickel, as described in detail in the Beggs patent.

In the foregoing description a number of the metal parts exposed to the interior of the device are of titanium. While these parts have advantages with respect to sealing and the like, they are also significant in the present device since the presence of titanium insures what may be termed a clean device, i.e. one in which the interior of the device remains relatively free from material which may be evaporated from parts of the device during operation. This results from the tendency of the titanium to sorb gases that are released, particularly at theelevated temperatures at which the device operates.

While the invention has been shown in a planar type device, it will be apparent that it is not limited to this particular configuration and it will also be appreciated that the grid may take various forms such as a mesh structure as long as the conductors of the grid are in alignment with the bare portions of the cathode and the coated portions of the cathode are in alignment with the grid openings.

While I have shown and described a particular embodimerit of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects and I aim, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device including an anode, a cathode, and an interposed control electrode supported in mutually spaced and insulated relation with respect to one another, said cathode including an area coated with electron emission enhancing material and a surrounding area of bare platinum with the surface of the platinum at least as high as the coating, said emitting area being in alignrncnt with an opening in said control electrode.

2. An electric discharge device including an anode, a cathode, and an interposed control electrode supported in mutually spaced and insulated relation with respect to one another, said cathode including an area coated with electron emission enhancing material and a surrounding area of bare platinum with the surface of the platinum at least as high as the coating, said emitting area being in alignment with an opening in said control electrode, said device including titanium metal exposed to the interior thereof to minimize the reevaporation of materials within said device during operating life thereof.

3. A bipotential cathode structure for an electric discharge device comprising a cathode electrode including areas of passive base metal with an electron emission enhancing coating thereon and exhibiting a relatively low work function and areas of bare platinum surrounding said coated areas and exhibiting a relatively high work function to provide a contact potential between said platinum areas and said emitting areas which focuses the electrons emitted from said focusing area.

4. An electric discharge device including an anode, a cathode, and an interposed control electrode supported in mutually spaced and insulated relation with respect to one another, said cathode including an area of passive base metal coated with electron emission enhancing material and a surrounding area of bare platinum with the surface of the platinum at least as high as said coating, said emitting area being in alignment with an opening in said control electrode, said device including metal parts of titanium exposed to the interior of the device to minimize the reevaporation of materials Within said device during operating life thereof.

5. An electric discharge device including an anode, a cathode, and an interposed control electrode supported in mutually spaced and insulated relation with respect to one another, said cathode including a platinum support including areas coated with electron emission enhancing material and surrounding areas of bare platinum with the surface of the bare platinum extending at least as high as the coated areas, said control electrode including a plurality of apertures with conducting boundaries, said emitting areas being in alignment with said apertures in said control electrode, and said device including titanium metal exposed to the interior thereof to minimize the reevaporation of materials within said device during operating life thereof.

No references cited, 

1. AN ELECTRIC DISCHARGE DEVICE INCLUDING AN ANODE, A CATHODE, AND AN INTERPOSED CONTROL ELECTRODE SUPPORTED IN MUTUALLY SPACED AND INSULATED RELATION WITH RESPECT TO ONE ANOTHER, AID CATHODE INCLUDING AN AREA COATED WITH ELECTRON EMISSION ENHANCING MATERIAL AND A SURROUNDING AREA OF BARE PLATINUM WITH THE SURFACE OF THE PLATIINUM AT LEAST AS HIGH AS THE COATING, SAID EMITTING AREA BEING IN ALIGNMENT WITH AN OPENING IN SAID CONTROL ELECTRODE. 